Cut butter into sifted flour and salt as for a pie crust; add sour cream andegg.,Cut through mixture with a pastry blender to mix thoroughly.,Press pastry into a ball.,______,Bake at 400F for 12 to 15 minutes, or until golden.,For filling: saute mushrooms andgreen onion in butter.,Blend in flour and salt; add cream.,Cook, stirring constantly, until thickened and smooth.,Fill pastry shells with mushroom mixture, garnish and serve.,Filled pastry shells can be frozen.,To thaw and heat, allow 12 minutes at 400F.